The increased density and reduced cost of integrated circuit components has made many previously impractical products commercially viable. One type of product made viable by advances in integrated circuit technology is the digital cordless telephone. A digital cordless telephone handset receives an analog speech signal via a microphone, converts the analog speech signal into a digital speech signal, compresses the digital speech signal, modulates the compressed signal at a radio frequency (RF), and transmits the modulated RF signal through an antenna. The transmitted RF signal is received by a nearby base station where it may be converted to an analog signal (or remain in digital form for digital switching equipment), and ultimately relayed to the destination telephone. When a similar signal is received from the destination telephone, the telephone signal undergoes the same process in reverse. The base station transmits a digital RF signal to the handset which is received via the antenna, demodulated, decompressed, and converted into an analog speech signal which drives a speaker in the handset.
One standard for the operation of a cordless telephone handset is the U.K. Cordless Telephone, Second Generation (CT-2) standard. A CT-2 handset receives an analog voice signal from a microphone. The voice signal is converted to digital pulse code modulation (PCM) format, processed through an adaptive differential pulse code modulation (ADPCM) encoder, and then modulated and transmitted according to the CT-2 Common Air Interface (CAI) protocol to a base station. CT-2 specifies CCITT G.721 ADPCM for the operation of the ADPCM encoder. Transmit and receive signals are sent and received to and from the base station in packets in a ping-pong fashion. CAI specifies that a part of the packet includes signalling information including such things as call setup and termination requests and handshaking information. For signal reception, a packet is received, demodulated, processed through a G.721 ADPCM decoder, converted from digital PCM to analog, and then provided to a speaker.
Both analog and digital functions are necessary in the operation of a digital cordless telephone handset. To perform the conversion from analog to PCM and from PCM to analog, a conventional integrated circuit such as the Motorola MC145554 .mu.-law PCM Codec-Filter or the Motorola MC145557 A-law PCM Codec-Filter may be used. To perform the conversion from PCM to ADPCM and from ADPCM to PCM, a conventional integrated circuit such as the Motorola MC145532 ADPCM Transcoder may be used. Both of these functions may also be combined into a single, mixed signal processing integrated circuit.
In the CT-2 cordless telephone system, the handset establishes a digital link with the base station. The link is normally maintained until the call is completed. In some cases, the link may be lost or broken between the handset and the base station during the call. For example, if the user moves the handset outside of the range of the base station, then the link may be lost. The result is that a high level of white noise is provided to the speaker due to the randomness of the received data until the broken link is detected and the noise is muted. A noise level of about 3 to 6 decibels (dB) below the maximum level may occur. The only method specified by the CAI protocol to detect link loss is to detect the absence of an expected handshake message in the signalling part of the packet. Intervals in the transmission of handshake messages may be as long as several seconds. One way to lessen the detection time is to force periodic communication between the base station and the handset, which results in a reduction in detection time to several hundred milliseconds. Even then, a user may hear a loud noise when the link is broken.